I have been thinking about getting a 1.4x teleconverter for my *istD. I was wondering what the difference (in AF performance, and image quality) between the Kenko (sp?) Sigma EX, and the Tamron teleconverters are. There is quite a large difference in price between the Sigma and the Tamron. Is the sigma worth the extra expense? The Sigma states that it is only compatable with cirtain Sigma and Pentax lenses. What happens if I use it on one that is not listed in the compatability list, like for example the SMC-FA 50 1.4?

I have also been considering a set of extention tubes for macro work (as I am not going to get a dedicated macro lense untill the D FA 100 comes out), are there any differences between the Kenko (sp?) and the Pentax ones. What do the different sizes mean? Does the higher the mm mean that you get greater magnification?